In recent years, the use of a wireless LAN operating at 2.4 GHz or 5 GHz has become more prevalent. Further, interest has been growing in, for example, a household electrical appliance home-network for performing mutual communications between household electrical appliances by using a small electric power wireless of a 400 MHz band or a 900 MHz band.
In the household electrical appliance home-network, household electrical appliances capable of performing wireless communications are relocated only a limited number of times. Further, in many cases, such household electrical appliances capable of performing wireless communications are installed in rooms having many communication obstacles, such as walls and floors are present. Because of these obstacles, a propagation path of the electric wave to be transmitted and received will be under the multipath environment, thereby constituting points at which the level of electric wave reception is reduced locally due to fading.
More specifically, as shown in FIG. 5, as a result of checking the state of a variation in the reception level of an electric wave by measuring the electric field intensities at positions (on the circumference of a circle) of the same distance from a transmitting unit under the multipath environment, it is clear that the reception levels are greatly different depending on positions as shown in FIG. 6. This is because of the occurrence of what is called a fading phenomenon in which electric waves are offset or added together by the multipath. FIG. 6 is a schematic view showing a layout example of a transmitting unit that is set under the multipath environment, and a receiving unit that is set on the circumference of a circle at the same distance from the transmitting unit. Further, FIG. 6 shows an example of a result of measuring electric field intensities at a plurality of points on the circumference of a circle shown in FIG. 5. FIG. 6 shows a correspondence relationship between the points on the circumference of a circle that are expressed on the lateral axis, and reception levels of electric waves that are expressed on the vertical axis.
As shown by the Rayleigh distribution in FIG. 7, the ratio for points, at which reception levels decrease by 8 dB or more from the median value of the reception levels, is 10%. Further, the ratio for points, at which reception levels decrease by 18 dB or more from the median value of the reception levels, is 1%. FIG. 7 is a probability distribution diagram showing an example of a relationship between a reception level and a probability that a wireless communication apparatus is present at the reception level. FIG. 7 is provided on the supposition that the reception level of the electric wave follows the Rayleigh distribution, assuming a state where attenuation of the electric wave is large due to the reflection by walls and the like.
In the case of a wireless terminal such as a mobile telephone that is used by being carried about, a communication environment changes following a movement, and therefore, there is a low possibility that the reception level stays at a locally declined reception level point for a long time. However, in the case of a wireless terminal that does not assume a movement such as a household electrical appliance that can perform wireless communications, a state of the declined reception level continues for a long time in some cases because of lack in the change in the communication environment.
In order to cope with such a state, there has been considered an attempt to improve qualities and reliability of communications by a selection diversity system that includes a plurality of antennas and preferentially uses a signal of an antenna which is in an excellent electric wave state among the same wireless signals that have been received by the plurality of antennas.
A plurality of antennas are used in the selection diversity system, but in the system since only one of the plurality of antennas is used after the selection of the antenna, antenna gain is no different from that of the configuration that does not employ this selection diversity system. Further, there may be a case at high probability where some of the plurality of antennas exhibit declined reception levels simultaneously, hence in some case the occurrence of declined levels cannot be sufficiently evaded.
Therefore, in order to evade the state of the declined reception level by improving the antenna gain, there has been a proposal to solve this problem by a maximum ratio combining diversity system that removes (reduces) noise by combining the same wireless signals that have been received by the plurality of antennas (refer to Patent Literature 1, for example).
That is, the combining circuit disclosed in Patent Literature 1 is configured to include a plurality of reception-wave weighting means corresponding to antennas, that perform weighting to input reception waves by the amplitude of the input reception wave and the inverse number of noise power, and a combining processing unit that combines the outputs of the reception-wave weighting means. Based on this configuration, even when noise power of each reception wave is different, a signal-to-noise power ratio (SNR, S/N ratio) of each combining reception wave can be maximized.
On the other hand, as a more simple method, there has been a proposal, by providing a plurality of antenna elements and a variable phase shifting unit, to perform communications by suitably correcting a phase shift quantity in the phase shifting unit that combines received reception signals based on so that the phase in the wave combining unit becomes a predetermined phase (refer to Patent Literature 2, for example). That is, a state of the declined reception level is avoided by performing communications when the phase shifting unit is set to a phase shift quantity at which the level of the combined wave is largest.
However, the conventional technique as disclosed in Patent Literature 1 has a problem in that a circuit configuration becomes large. More specifically, the combining circuit disclosed in Patent Literature 1 is configured to include a receiving circuit and a demodulation processing circuit for each antenna (for example, an envelope line estimating unit, a residual power estimating unit, a residual power calculating unit, a complex multiplying unit, a multiplying unit, and a combining unit). Particularly, a wireless communication apparatus that is incorporated in the household electrical appliance and like is required to have a miniaturized size, a simplified circuit configuration, and reduce electric power consumption. As in the conventional art, if the number of the receiving circuits and demodulation circuits to be provided in the configuration has to be predicated on the number of antennas, it is difficult to achieve miniaturization.
The conventional technique as disclosed in Patent Literature 2 has a problem in that the setting of a variable phase shifting unit needs to be searched for each time communications are performed with a different household electrical appliance in a situation where communication is performed among a plurality of household electrical appliances, and also has a problem in that certain time is required until reception is implemented.